Coking of spent pulping liquors used in the alkaline sulfite process

ABSTRACT

Presented is a process wherein spent pulping liquors used in the Alkaline Sulfite Process for pulping wood and having a pH of about 9 to 12 are coked in the liquid phase in a coking zone at about 450*F to 750*F and autogenous pressure of 100 to 3000 psig for 0.5 minute to 6 hours. The gases, liquid effluent and coke which form are separated. The coke is washed for removal of salts and dried with flue gas. Next, the dried coke and the gas are burned to produce steam, supply heat for coking and to generate SO2 in the flue gas. Some of the effluent is flashed in a flashing zone to produce water for washing the coke followed by heat exchange of the bottoms from the flashing zone with the spent liquors to preheat the same. Other portions of the effluent are recombined and reconstituted for use as pulping liquor. No pH reduction is necessary prior to coking and the COD of the effluent is normally about 75 percent less than that of the starting liquors.

United States Patent Hess et al.

COKING OF SPENT PULPING LIQUORS USED IN THE ALKALINE SULFITE PROCESS Inventors: Howard V. Hess, Glenham; Edward L. Cole, Fishkill; William F. Franz, Gardiner, all of NY.

[73] Assignee: Texaco Inc., New York, NY.

[22] Filed: Apr. 25, 1972 [21] App]. No.: 247,379

[52] US. Cl 162/31; 162/36 [51] Int. Cl D21c 11/02; C210 11/14 [58] Field of Search 162/30, 36, 31; 423/207 [56] References Cited UNITED STATES PATENTS 3,595,806 7/1971 Prahacs et al. 162/30 X 3,607,619 9/1971 Hess et a1 162/30 3,650,888 3/1972 Fogman et a1. 162/30 3,705,077 12/1972 Franz et a1 162/30 3,717,545 2/1973 Hess et al. 162/31 Primary Examiner-S. Leon Bashore Assistant ExaminerAlfred DAndrea, Jr.

Attorney, Agent, or Firm-T. H. Whaley; C. G. Ries; Henry W. Archer [57] ABSTRACT Presented is a process wherein spent pulping liquors used in the Alkaline Sulfite Process for pulping wood and having a pH of about 9 to 12 are coked in the liquid phase in a coking zone at about 450F to 750F and autogenous pressure of 100 to 3000 psig for 0.5 minute to 6 hours. The gases, liquid effluent and coke which form are separated. The coke is washed for removal of salts and dried with flue gas. Next, the dried coke and the gas are burned to produce steam, supply heat for coking and to generate S in the flue gas. Some of the effluent is flashed in a flashing zone to produce water for washing the coke followed by heat exchange of the bottoms from the flashing zone with the spent liquors to preheat the same. Other portions of the effluent are recombined and reconstituted for use as pulping liquor.

No pH reduction is necessary prior to coking and the COD of the effluent is normally about 75 percent less than that of the starting liquors.

2 Claims, 2 Drawing Figures 1 22 LIME IIEE'KEINE 2O Z3COOLING U ADDITION SULFITE 24 LIQUOR 2\ 25 WATER CONDENSE 147 1e HOLD 33 v 1 DRUM FLASH 42 44 (O 5 DRUM 27- 26' Q 4| PLANT 32 j STEAM cAusTlcm 7 30 3| 3e g 43 TO PULPING DIGESTORS COKE WASHER a 432 z 2 x 34 CLARIFIER 46 COKE DRBYBER FILTER sumeor 2 ALKALINE SULIFITE LIQUOR 600F "woop g I/ so sso|= llOOpsug COD REMOVA L PATENTED HAY 2 I975 MINUTES OF TF I0 3o so so a0 I00 no I20 AUTOCLAVE' ZHRS.

1 COKING OF SPENT PULPING LIQUORS USED IN THE ALKALINE SULFITE PROCESS BACKGROUND OF THE INVENTION This invention relates to the treatment of spent liquors used in the Alkaline Sulfite Process for pulping wood.

The above-mentioned process is described in Canadian Pat. No. 847,218, issued July 21, 1970 to Canadian International Paper Company. In this process the hot pH" of the cook is maintained at around 9-12 during the cooking by adding alkali thereto. Conventional sulfite pulping is carried out well on the acid side or, more recently, under slightly alkaline conditions of up to pH 9.

A competitive process, the Kraft pulping process, operates in the same alkaline pH range of 9-12 as does the alkaline sulfite pulping process. Alkaline sulfite pulping combines the advantages of Kraft and conventional sulfite pulping while minimizing the disadvantages associated with each. The brighter pulp and minimal atmospheric emission of maladorous sulfur compounds associated with sulfite pulping are combined with the higher strength associated with Kraft pulp. Disadvantageously, however, no commercial process exists for disposal of or chemical recovery from the spent pulping liquors of the alkaline sulfite process in contrast to the extensive spent Kraft liquor processing technology. The present process provides such a means of handling spent alkaline sulfite pulping liquors.

BRIEF SUMMARY OF THE INVENTION dried with flue gas. Next, the dried coke and the gas are burned to produce steam, supply heat for coking and to generate S in the flue gas. Some of the effluent is flashed in a flashing zone to produce water for washing the coke followed by heat exchange of the bottoms from the flashing zone with the spent liquors to preheat the same. The portions of the effluent are recombined and reconstituted to supply pulping liquor to the pulping operating.

No pH reduction is necessary prior to coking and the COD of the effluent is normally about 75 percent less than that of the starting liquors.

BRIEF DESCRIPTION OF THE DRAWINGS The process of this invention will be better understood by reference to the accompanying drawings in which: I

FIG. 1 illustrates diagrammatically a preferred embodiment of the present invention; and

FIG. 2 illustrates a curve showing COD removal versus time whencoking alkaline sulfite liquor.

, DESCRIPTION OF PREFERRED EMBODIMENT In the physical embodiment of the invention diagrammed in the drawing, spent alkaline sulfite liquor is raised to coking pressure of 1000-1 IOO psig by pump and preheated through exchanger 11, flowing to fired heater 12 where its temperature is raisedto the the liquid phase effluent passes through line to point coking temperature of 550F. The heated liquor then flows through line 14 to the coker hold drum .15, which provides the necessary residence time for completion of the coking reactions as well as some settling of the coke particles to produce a clear effluent which leaves the hold drum through line 16. Upon pressure reduction through valve 17, a portion of the effluent is vaporized and separated from the vaporized liquid in flash drum 18. The unvaporized liquid flows from flash drum 18 through line '19 and cooled by heat exchange with incoming feed in exchanger 11. From exchanger 11,

21 where it is combined with other liquid streams from the process.

That portion of the clear coker effluent which was vaporized in flash drum 18 passes to surface condenser 22. Fixed gases are taken overhead through line 23 and reduced to substantially atmospheric pressure by control valve 24. At point 25 they are combined with depressurized gas produced by the coking reactions taking place in hold drum 15. The combined gas stream is then brought to the fired heater 12 through line 26 where sulfur-containing compounds as well as other combustible gases are incinerated.

Condensate from surface condenser 22 is conveyed by line 27 to coke washer 28 where sodium ions and other pulping chemicals retained on the coke are recovered by washing. From coke washer 28, the condensate stream, now containing chemicals recovered from the coke, passes through line 29 and is combined at point 21 with the liquid phase material from which it was separated in flash drum 18.

A settled coke slurry is removed from the bottom of hold drum 15 through line 30 and dewatered in dewatering device 31. Device 31 may be a liquid cyclone or other suitable device for solid-liquid separation. Overflow passes to coke washer 28. Here, as described previously, condensate from line 27 washes occluded sodium and other pulping chemicals from the coke. Washed coke settles to the bottom of 28 and is again dewatered in dewatering means 34, a liquid cyclone or other device similar to 31. Overflow from 34 joins the main washings stream through line 35 at point 36. Dewatered-coke is carried byline 37 to coke dryer 38 where hot flue gas from fired heater 12 through line 48 evaporates residual water from the coke to produce a dry granular solid. The dried coke is removed from the 7 bottom of dryer 38 and conveyed by line 49 to fired hheater 12 where it is burned to provide process heat for the cokingoperation as well as to raise steam for general plant use.

Flue gas exits from dryer 38 through line 39 and passss to S0 recovery absorber 40. Here sulfur dioxide generated by the combustion of sulfur-containing coke from line 49 and the sulfur compounds contained in the combined gas stream in line 26 are absorbed in the combined liquid effluent from coking, which is charged to absorber 40.through line 41. Pollutant-free flue gas exits from absorber 40 through line 42 and is vented to the atmosphere. The liquid effluent passes from absorber 40 to causticizer 44 through line 43. In causticizer 44 lime is added to convert sodium carbonate, produced by reaction with CO from the coking reaction, to sodium hydroxide required for the high pH sulfite pulping liquor. Removal of lime mud in clarifier 45' and in filter 46 results in a reconstituted pulping liquor which is recycled to the digesters through line 47. Clarflue' "gascontactin'g for S02 recovery, 'converts'NaOH ent from the coker it reacts preferentially with NaOH rather than Na CO Thus the effluent from the coker, either before or after absorption'of S should be causticized, preferably with lime, to bring the final fortified present in the spent liquor to Na CO and/or NaHCO;;. 5 coker effluent up to a pH suitable for alkaline sulfite Addition of lime then converts these carbonates to hy- WOOd pulpmg.

droxide with the formation of CaCO The latter is in- The graph of FIG. 2 has a curve showing COD rel bl d must b removed prior t using the o moval against time when coking alkaline sulfite liquor stituted pulping liquor. at. 550]: and 1 100 p 1 In an example of the successful practise of the invenifnother example of the described P P tion, spent Alkaline Sulfite Process liquor was coked at Alkalme Process q fwas coked at 600}: 550F under 1 100 psig for 2 hours with the results tabuunder 1700 P 2 for 2 hours with the results tabulated lated below. below- TABLE 1 Wt?! Wt'/r WtVr WtVr WtVr WtVl Wt'/1 Wt /I Wt?! Gross Heat Yield Dissolved COD Ash C Sulfur Na ca N H- pH of Solids g/l Combustion BTU/1h Spent Liquor 20.3

Effluent from Coker 80.4 12.2 54.5 8.49 2.5 1.45 3.9 0.05 0.005 9.4 Wet Coke 17.1 (74.8% H O) Dry Coke 2.14 28.18 59.8 4.1 8.1 0.14 3.9 9,880(9,9 83) Washed Dry Coke 1.88 4.23 74.7 3.7 1.9 0.15 4.2 12.219(12.359) Gas 0.53 M01 26.4 12.6

Carbon Dioxide 70.5

Thiophene 0.2

Dimethyl Sulfide 2.5

Methyl Mercaptan 2.3 MW 38.2

Hydrogen Sulfide 9.8

Ethane 0.9

Methane 3.1

Hydrogen 10. 8

TABLE 2 Wt 7r Gross Heat Wt 7( Dissolved COD Wt /1 Wt 'Ir Wt 71 Wt /1 Wt 71 Wt Wt /1 of Yield Solids g/l Ash C Sulfur Na Ca N H; pH Combustion BTU/1h Spent Liquor 20.3

Effluent from Coker 78.6 12.1 37.0 8.0 2.0 1.46 3.7 0.004 9.2

Wet Coke 17.2 (717: H O) Dry Coke 5.0 l 27.85 68.4 5.5 9.9 0.07 3.8 10.245

Washed Coke 3.52 7.45 77.1 3.8 0.9 0.09 3.9 13,191

Gas 0.67 M01 7r 34.5 5.7

Dimethyl Sulfide 1.3 Methyl Mercaptan 1.5 'CO- 64.6

H 8 3.0 MW 32.4 Ethane 1.2 Methane 3.5 Hydrogen 23.8

The gas produced is high in carbon dioxide, produced no doubt'from decarboxylation of wood sugars and the like. The washed dry coke produced normally has a gross heating value of around 12,200 BTU/lb and usually contains about 3.7 percent-sulfur. This coke is burned in a heater to produce high pressure process steam and also to supply heat for the liquid phase coking. Sulfur dioxide is contained in the flue gases from the heater and absorbed in the coker effluent to make sodium sulfite (a pulping chemical). The fortified effluent with such added chemical fortification as required When $0: (from the flue gas) is absorbed in the efflu- What is claimed is:

1. In the coking of spent pulping liquors used in the Alkaline Sulfite Process which have a pH of about 9 to about 12; the steps of:

"coking said liquors in the liquid phase in a coking zone at about 450F. to about 750F. under a pressure of about to about 3000 psig for about 0.5 minute to about 6 hours to form gases including CO liquid effluent and coke; separating said gases, said effluent and said 7 coke; flowing said gases to a combustion zone wherein malodorous sulfur compounds are converted to S0 vaporizing a portion of said effluent and cooling the remainder by heat exchange with incoming liquor to preheat said liquor; flowing said vaporized portionof said effluent to a condensing zone; condensing said portion to form a condensate; washing said coke with said condensate to remove sodium ions and other v 6 thereby; flowing said liquid stream to a causticizing zone, and

adding lime to said stream to convert sodium carbonate produced therein by reaction with CO pro duced by said coking to NaOH and clarifying said stream to remove solids resulting from causticizing, thereby forming new cooking liquor having a pH suitable for alkaline sulfite pulping.

2. The process of claim 1 including the steps of dewatering said coke after washing same. 

1. IN THE COKING OF SPENT PULPING LIQUORS USES IN THE ALKALINE SULFITE PROCESS WHICH HAVE A PH OF ABOUT 9 TO 12; THE STEPS OF: COKING SAID LIQUORS IN THE LIQUID PHASE IN A COKING ZONE AT ABOUT 450*F. TO ABOUT 750*F. UNDER A PRESSURE OF ABOUT 100 TO ABOUT 3000 PSIG FOR ABOUT 0.5 MINUTE TO ABOUT 6 HOURS TO FORM GASES INCLUDING CO2, LIQUID EFFLUENT AND COKE; SEPARATING SAID GASES; SAID EFFLUENT AND SAID COKE; FLOWING SAID GASES TO A COMBUSTION ZONE WHEREIN MALODOROUS SULFUR COMPOUNDS ARE CONVERTED TO SO2; VAPORIZING A PORTION OF SAID EFFLUENT AND COOLING THE REMAINDER BY HEAT EXCHANGE WITH INCOMING LIQUOR TO PREHEAT SAID LIQUOR; FLOWING SAID VAPORIZED PORTION OF SAID EFFLUENT TO A CONDENSING ZONE; CONDENSING SAID PORTION TO FORM A CONDENSATE; WASHING SAID COKE WITH SAID CONDENSATE TO REMOVE SODIUM IONS AND OTHER PULPING CHEMICALS RETAINED ON SAID COKE; COMBINING SAID CONDENSATE NOW CONTAINING CHEMICALS RECOVERED FROM SAID COKE, WITH THE COOLED EFFLUENT AND CONVEYING THE COMBINED STREAM TO AN SO2 -CONTAINING ABSORBING ZONE; DRYING SAID COKE WITH FLUE GAS, BURNING SAID COKE IN A COMBUSTION ZONE TO PROVIDE USEFUL HEAT, TO INCINERATE AFOREMENTIONED SULFUR-BEARING GASES AND TO GENERATE AN SO2BEARING FLUE GAS; THEN FLOWING SAID FLUE GAS TO SAID SO2 ABSORBING ZONE; VENTING OFF THE POLLUTANT-FREE FLUW GAS OBTAINED THEREBY; FLOWING SAID LIQUID STREAM TO A CAUSTICIZING ZONE, AND ADDING LIME TO SAID STREAM TO CONVERT SODIUM CARBONATE PRODUCED THEREIN BY REACTION WITH CO2 PRODUCED BY SAID COKING TO NAOH AND CLARIFYING SAID STREAM TO REMOVE SOLIDS RESULTING FROM CAUSTICIZING, THEREBY FORMING NEW COOKING LIQUID HAVING A PH SUITABLE FOR ALKALINE SULFITE PULPING.
 2. The process of claim 1 including the steps of dewatering said coke after washing same. 